Image forming apparatuses utilizing digital techniques, for example, electronic copiers, have a fixing apparatus that fixes a thermally melted development image on a sheet under pressure.
The fixing apparatus comprises a heating roller that melts a developer, for example, toner, and a pressurizing roller that exerts a predetermined pressure on the heating roller. A predetermined contact width (nip width) is formed in a contact area (nip portion) between the heating roller and the pressurizing roller. A development image on a sheet melted by heat from the heating roller is fixed on a sheet passing through the nip portion, under pressure from the pressurizing roller. In recent years, an induction heating device has been utilized which forms a thin metallic conductive layer outside the heating roller and which heats the metallic conductive layer using induction heating.
A known method for the induction heating device uses a detection element that is brought into contact with the surface of the heating roller to detect the temperature of the surface. The method thus controls the induction heating of the heating roller on the basis of the detected temperature. However, this contact temperature detecting element may degrade the surface of the heating roller when sliding on it. This may disadvantageously reduce the lifetime of the heating roller. The degraded surface of the heating roller may also reduce the responsiveness of the temperature detecting element, which may thus incorrectly detect a target temperature.
A known technique uses a temperature detecting element that detects infrared radiation emitted by the heating roller to determine the temperature of the heating roller in a non-contact manner. This non-contact temperature detecting element condenses infrared radiation from the target via a condensing lens to detect the target temperature on the basis of the quantity of infrared radiation received. This enables the surface temperature to be detected without damaging the heating roller.
However, toner or paper dust flaying in the fixing apparatus may disadvantageously contaminate a lens of the temperature detecting element. The contaminated lens may reduce the quantity of infrared radiation received by the temperature detecting element. This may result in an error in the value detected by the temperature detecting element.
For example, an image forming apparatus is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-34109. This image forming apparatus measures variations in the output characteristics of thermopiles 203 and 204 with respect to variations in outputs from thermistors 205 and 206 that detect the self temperatures of the non-contact temperature detection sensors 39c and 39d. On the basis of these variations, the image forming apparatus detects contamination of the surface of the sensor to compensate for a detected temperature characteristic depending on the condition of the contamination.
Jpn. Pat. Appln. KOKAI Publication No. 2003-4536 discloses a fixing apparatus comprising a movable filter 4. The movable filter 4 is prepared free movement between a heated member 1 that is a temperature detection target and a detection surface 3a of a non contact temperature sensor 3 placed opposite the heated member 1, and allows infrared radiation to pass; the infrared radiation is emitted by the heated member 1.
Jpn. Pat. Appln. KOKAI Publication No. 10-31390 discloses an electrophotographic apparatus that controls the temperature of a heating roller 9 on the basis of a sensing output from a non-contact temperature sensor 14. In this apparatus, the non-contact temperature sensor 14 has self temperature sensing means for providing sensing outputs based on the difference between its self temperature and the temperature of a heating roller that is a target. When a sensing output from the self temperature detection means is defined as T0 and a self temperature output is defined as T1, the temperature T of the heating roller is controlled on the basis of its value, that is, on the basis of a multi-order equation of T1: T=C(T1)+f(T1)×T0+g(T1)×T0^2+h(T1)×T0^3+ . . . and a function expression of T1: C(T1), f(T1), g(T1), h(T1), . . . (example: f(T1)=constant A+α×T1+β×T1^2+γ×T1^3+ . . . ) (constants A, α, β, and γ are real numbers other than 0).